Filter Exposure understanding help!

[Shaggysk8]

Hello,

OK for B&W photography I am gonna now start using some filters, I just wanted to ask a quick question.

So if I attach a orange filter to my camera lens and I should adjust my exposure by say 2 stops, now that is fine I get that. But I have confused my self by thinking ok what is happening, is it a) the orange filter is letting more orange light in and turning my blue, lets just say 2 stops from what it should be or b) it is stopping all light from coming in by 2 stops.

If it is a, then does that mean you might not always want to adjust exposure?

Paul

[richard ide]

The filter reduces the amount of light reaching the film so exposure must be increased to compensate. The filter modifies the spectrum of light hitting the film but also reduces the light intensity.

[Shaggysk8]

Ok so not only does it stop the amount of light it will also stop, say the blue sky down as well?

Just need to get it in my head as I am off out tomorrow

[Rick A]

First, if your camera is set to auto mode, the TTL metering will automatically compensate for the filter factor, most cameras will compensate up to two or three stops. On manual mode, you meter without the filter, then open the amount of stops depending on the specific factor for that filter. For a Y2 (yellow) filter , factor is 2, so open one stop(2 is double the exposure). R(25A) red filter has a factor of 8( 8 times the exposure needed) so you open 3 stops. Are you confused yet? Eight times the exposure is : open 1 stop(x2) open another stop(x2 again or 4x the original), open one more stop(x2 again or8x the original) so we have 2x2x2, or 8x the original meter reading. I keep a cheat sheet of factors, plus each factor taped to the filter case, also, I have a chart that tells what filter to use for whatever correction needed.

Rick

[Q.G.]

A filter reduces the amount of light that comes through (never increases the amount of light), but differently for different colours.
You compensate to get the overall exposure right, but the difference in balance between colours the filter produces remains.

[Kevin Kehler]

The orange filter is letting in more "orange" light but is also blocking out more blue and green light. Since orange is composed of yellow and red, it affects both of those colours as well. So in the picture of the red truck in a yellow wheat field under a green tree and blue sky, the wheat and truck will become lighter (i.e. whiter) while the tree and sky will both become darker (i.e. blacker). Coloured filters have little-to-no effect on gray.

It is stopping more light from reaching the film since you have added an additional surface of glass. One thing to note is that different filter brands have different filter factors: my yellow filter needs only a half stop increase (should be 1 full stop) and my red filter needs a 3 and a half stop increase (should be 3).

[paul_c5x4]

Just need to get it in my head as I am off out tomorrow

I'm sure someone will refer you to Ansel Adams' The Negative and the chapter on filters.. In the meantime, Kodak have a publication covering filters & Mr Adams refers to a "Color Filter Wheel". Just for you, I found a copy of the document here, or if you prefer a colour rendition, Fig.7 of this.

[pentaxuser]

You can check the accuracy of the TTL metering vis a vis the filter factor by leaving the camera on a scene and noting the exposure then placing the filter in front and note it again. If the filter is a 2 stop filter but the TTL registers more or less than 2 stops then the TTL isn't registering the filter factor accurately.

You then have to decide if overriding the TTL meter and using the filter factor is worth the effort.

pentaxuser

[Sim2]

The filter will lighten objects that are the same colour as the filter.
The filter will darken objects that are the opposite colour as the filter.

e.g. a red filter will lighten anything red in the scene but darken anything blue in the scene.

The filter factor is a combination of the strength of the filter i.e. pale red, dark red and the proportion of the light that the filter would stop being transmitted to the film under average daylight conditions.

The filter stops some part of the spectrum of light reaching the film so exposures have to be increased proportionately.

Paul_c5x4s suggestion of the colour wheel is a brilliant one - look and learn!

[Q.G.]

The orange filter is letting in more "orange" light but [...]

It is only "letting in more" orange in the sense that it doesn't stop as much orange as, say, blue.

So that wheat and that truck from your example do not get lighter.
They get darker too. Hence the compensation.
But they don't get as dark as the sky, so despite compensation [you get the picture].

It is stopping more light from reaching the film since you have added an additional surface of glass.

If uncoated, some 4% per surface (and you of course add two, not one) lost to reflection.
Not much.